A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In recent developments, an increased problem of providing thermally stable substrates occurs due to ever more demanding requirements for image resolution, such as in the new field of immersion lithography. Here, an increased problem of thermal stabilization occurs, because immersion liquids may cause thermal cooling by transitioning to a vapor phase. Hence, it is found that local thermal gradients may occur in the substrate, and such gradients should be stabilized. Also, it has been found that during a photolithographic treatment of a substrate, the substrate, due to subsequent illuminations of target portions, tends to warm up unevenly, with a temperature front that progresses over the surface area of the substrate. This temperature front may also cause thermal gradients, which may amount to local deformations of the substrate. Evidently, for nanometer projection accuracy, this may cause problems in focus and overlay errors. That is, due to thermal deformations, the surface of the substrate may bend away from an ideal projection plane, which may result in focus loss or at least an effective sideways movement of the image plane such that overlay problems may occur.